1. Field of the Invention
The present invention relates to a polishing composition to be used for polishing substrates for semiconductors, photomasks and various memory hard disks, particularly to a polishing composition useful for polishing for planarization of the surface of device wafers in e.g. semiconductor industry, and a polishing method employing such a composition.
More particularly, the present invention relates to a polishing composition which is highly efficient and provides high selectivity in the polishing of semiconductor devices to which so-called chemical and mechanical polishing (CMP) technology is applied, in the processing of device wafers, and a polishing method employing such a composition.
2. Discussion of Background
Progress of so-called high technology products including computers has been remarkable in recent years, and parts to be used for such products, such as ULSI, have been developed for high integration and high speed, year after year. Along with such progress, the design rule for semiconductor devices has been progressively refined year after year, the depth of focus in a process for producing devices tends to be shallow, and planarization required for the pattern-forming surface tends to be increasingly severe.
Further, to cope with an increase in resistance of the wiring due to refinement of the wiring, it has been studied to employ copper instead of tungsten or aluminum, as the wiring material.
By its nature, copper is hardly processable by etching, and accordingly, it requires the following process. Namely, after forming wiring grooves and vias on an insulating layer, copper wirings are formed by sputtering or plating, and then an unnecessary copper layer deposited on the insulating layer is removed by chemical mechanical polishing (hereinafter referred to as CMP) which is a combination of mechanical polishing and chemical polishing.
However, in such a process, it may happen that copper atoms will diffuse into the insulating layer to deteriorate the device properties. Therefore, for the purpose of preventing diffusion of copper atoms, it has been studied to provide a barrier layer on the insulating layer having wiring grooves or vias formed. As a material for such a barrier layer, metal tantalum or a tantalum-containing compound (hereinafter will generally be referred to as a tantalum-containing compound) is most suitable also from the viewpoint of the reliability of the device and is expected to be employed mostly in the future.
Accordingly, in such a CMP process for a semiconductor device containing such a copper layer and a tantalum-containing compound, firstly the copper layer as the outermost layer and then the tantalum-containing compound layer as the barrier layer, are polished, respectively, and polishing will be completed when it has reached the insulating layer of e.g. silicon dioxide or silicon trifluoride. As an ideal process, it is desired that by using only one type of a polishing composition, the copper layer and the tantalum-containing compound layer are uniformly removed by polishing in a single polishing step, and polishing will be completed certainly when it has reached the insulating layer. However, copper and a tantalum-containing compound are different in their hardness, chemical stability and other mechanical properties and accordingly in the processability, and thus, it is difficult to adopt such an ideal polishing process. Accordingly, the following two step polishing process, i.e. polishing process divided into two steps, is being studied.
Firstly, in the first polishing step (hereinafter referred to as the first polishing), using a polishing composition capable of polishing a copper layer at a high efficiency, the copper layer is polished using e.g. a tantalum-containing compound layer as a stopper until such a tantalum-containing compound layer is reached. Here, for the purpose of not forming various surface damages such as recesses, erosion, dishing, etc., on the copper layer surface, polishing may be terminated immediately before reaching the tantalum-containing compound layer i.e. while a copper layer still slightly remains. Then, in the second polishing step (hereinafter referred to as the second polishing), using a polishing composition capable of polishing mainly a tantalum containing layer at a high efficiency, the remaining thin copper layer and the tantalum-containing compound layer are continuously polished using the insulating layer as a stopper, and polishing is completed when it has reached the insulating layer.
The polishing composition to be used in the first polishing is required to have a property such that it is capable of polishing the copper layer at a high stock removal rate without forming the above-mentioned various surface defects on the copper layer surface, which can not be removed by the second polishing.
With respect to such a polishing composition for a copper layer, for example, JP-A-7-233485 discloses a polishing liquid for a copper type metal layer, which comprises at least one organic acid selected from the group consisting of aminoacetic acid and amidesulfuric acid, an oxidizing agent and water, and a method for producing a semiconductor device using such a polishing liquid. If this polishing liquid is used for polishing a copper layer, a relatively high stock removal rate is obtainable. It is believed that copper atoms on the copper layer surface be oxidized by the action of the oxidizing agent, and the oxidized copper elements are taken into a chelate compound, whereby a high stock removal rate can be obtained.
However, as a result of the experiments conducted by the present inventors, it has been found that in polishing a semiconductor device comprising a copper layer and a tantalum-containing compound layer, the polishing liquid of JP-A-7-233485 is effective as a polishing composition mainly for a copper layer i.e. for the first polishing, but it is hardly useful mainly for a tantalum-containing compound layer i.e. for the second polishing, because the tantalum-containing compound is susceptible to oxidation by an oxidizing agent such as hydrogen peroxide, an iron salt or even water, and besides, the oxidized layer tends to be very hard and can not easily be polished. Namely, this oxidized layer (ditantalum pentoxide) is a very hardly polishable material, whereby polishing will not proceed even by polishing by means of a large amount of an oxidizing agent. Further, there has been a problem that due to the strong ferroelectricity of such oxidized layer, self-discharge is likely to occur during polishing, whereby explosive chipping of the tantalum-containing compound layer is likely to occur frequently. Whereas, a copper layer is relatively easily polished, and the stock removal rate against the copper layer is higher than the stock removal rate against the tantalum compound, whereby surface damages such as recesses, dishing, erosion, etc., have been likely to form.
On the other hand, a method of mechanically polishing a tantalum-containing compound layer without relying on the chemical action, is also being studied. In a polishing composition to be used for this purpose, a relatively hard abrasive such as aluminum oxide, silicon nitride or silicon carbide may be used, and the content of such an abrasive may be increased. However, if a composition containing a large amount of such an abrasive, is used for polishing, although the stock removal rate against the tantalum-containing compound layer may increase to some extent, the stock removal rate against the insulating layer also increases at the same time. Accordingly, the ratio (hereinafter referred to as "the selectivity ratio") of the stock removal rate of the insulating layer to the tantalum-containing compound layer, tends to be very small, and consequently, the yield of the device tends to decrease. Further, if such a composition is used for polishing, the usage life of the polishing pad is likely to be shortened, thus leading to a problem from the viewpoint of the production cost.